The invention relates to an arrangement for producing pulse-shaped signals whose amplitude is determined by a control voltage. The arrangement comprises a first transistor to be operated as a current source having an output electrode, a main electrode and a control electrode. The control voltage can be applied across the input circuit of said transistor. The arrangement includes an output terminal which is coupled to the output electrode of said transistor and means for transferring the current which is determined by the control voltage to the output terminal in a pulsating fashion. In this respect output electrode, main electrode and control electrode are to be understood to mean those electrodes which in the case of a bipolar transistor are called the collector electrode, emitter electrode and base electrode respectively and in the case of a field-effect transistor the source electrode, the drain electrode and the gate electrode respectively. Input circuit is to be understood to mean the circuit across the control and main electrode, which circuit may include impedances.
Arrangements of the type mentioned in the preamble are inter alia employed in electronic organs. The control voltage is then applied upon depression of a specific key so that the arrangement supplies a pulse-shaped signal with a specific amplitude and frequency, which signal can be rendered audible with a loudspeaker. Upon release of the key the control voltage gradually decreases so that the corresponding tone gradually decays.
An arrangement of the type mentioned in the preamble but for a different application, is known from "Neues aus der Technik", No. 2, page 1, Apr. 1, 1970, article No. 926. In this arrangement a voltage is applied to the base electrode of the first transistor whose emitter electrode is connected to a point of fixed potential via a resistor. The collector electrode is connected to the common emitter electrodes of a second and a third transistor. The collector electrode of the second transistor constitutes the output, and a switching voltage is applied to the base electrode of the third transistor. Owing to this switching voltage the second and the third transistors are alternately conductive so that the current which is produced by the first transistor is transferred to the output in a pulsating fashion.
In another arrangement known from the literature (FIG. 1) for producing pulse-shaped signals with an amplitude which is determined by the control voltage, the means for transferring the current which is determined by the control voltage to the output terminal in a pulsating fashion comprise a voltage source for producing pulse-shaped voltages. This voltage source is in series with an impedance and is included between the emitter of the first transistor and a point of fixed potential. The control voltage is then applied to the base. If the voltage of the voltage source is high the transistor is reverse-biased and if the voltage of the voltage source is zero volts, the collector current of the transistor is determined by the control voltage. Thus, a pulse-shaped current also is obtained whose amplitude is determined by the control voltage.
The arrangements of the above-mentioned type all supply a unipolar signal. As a result, the pulse-shaped output current has a d.c. component. This d.c. component, which is not suppressed at once, causes an audible switching click upon depression of a key. When employing such arrangements in integrated circuits and in the case of application in low frequency circuits it is difficult to filter out said d.c. component with the aid of isolating capacitors. In addition, there is the problem that in the case of electronic organs the capacitance of the isolating capacitor should be sufficiently high to allow low frequencies of, for example, 25 Hz to pass through. As a result of this, the capacitor is charged comparatively slowly by the d.c. component of a pulse train which is produced, so that the first pulses of a train still comprise substantially the full d.c. component, which gives rise to amplifiers and loudspeakers being overdriven sooner.
It is an object of the invention to provide an arrangement of the type mentioned in the preamble which is capable of producing a pulse-shaped output signal without a d.c. component.
In the case of pulse-shaped signals whose relative pulse width is 0.5, the absence of the d.c. component also results in a symmetrical bipolar signal being obtained. This is not the case with pulse-shaped signals with a different relative pulse duration. As a conversion of unipolar signals into symmetrical bipolar signals may often be desirable. it is a further object of the invention to provide an arrangement of the type mentioned in the preamble which is capable of supplying a symmetrical bipolar output signal.